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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorAMG
    • CommentTimeJun 7th 2021
    We are due to start renovating a 1.5 storey bungalow that had previously had the loft converted to bedrooms by creating an 'insulated' box within the roof truss space. The insulation work is awful with 75mm rigid board celotex between framework that are falling about all over the place with no taping or any other barrier in sight. There is plenty of cold air blowing straight in through the continuous soffit vents.

    We want to remove the room in a box and bring in all the useable space into the thermal envelope.
    The roof from what I can see is rafters with sarking boards and then some form of bitamin/thick felt followed by cross battens and then tiles.

    1. What insulation material would you advice to minimise heat loss but also avoid a large build up as ceiling height is tight?
    2. Can you insulate to full depth of rafter (all the way to the sarking board) or do you need to leave any gaps?
    3. Presumably will need a VCL to stop moisture condensing on the sarking board?
    3. Is it worth stripping roof and then insulating on top of sarking board? Not sure if I can get away without needing planning due to increase in height?? Various veluxes will also have to be refitted.

    How would you tackle the above and what insulation material would you suggest? BTW we are in coastal Scotland and exposed to the wind.

    Much appreciated and many thanks for your advice/suggestions.
    Best solution would be to turn it into a warm roof , with insulation on top.
    Quite a bit of work unless you plan to reroof anyway.
    I use PUR or similar. Not sure how that qwould work in your area though.
    Could you stick OSB sarking board below as VCL
    and another on top to aid with fixing battons and wind load issues?
    creating something like your own SIP roof panel.
    • CommentAuthorEd Davies
    • CommentTimeJun 7th 2021 edited
    Posted By: AMG2. Can you insulate to full depth of rafter (all the way to the sarking board) or do you need to leave any gaps?
    I'll be insulating to full depth though that needed a discussion with the BCO. NE Scotland (Caithness).

    235mm I-beam rafters with 22x150mm sarking boards over (then breathable membrane, counter battens, battens, profile steel sheeting). I'll full fill between the rafters with mineral wool then have PUR below and across them with a vapour barrier.

    The house designer said I'd need a 50mm or so gap below the sarking boards before the mineral wool but the BCO was open to the point that mineral wool is sufficiently vapour open that, with all points within 75mm of a gap in the boards any airspace there wouldn't actually contribute anything, particularly as there's a 50mm space over the top. Something less vapour open than mineral wool would probably require ventilation below the sarking boards, though.

    How would you tackle the above and what insulation material would you suggest?
    Same as I'm doing on my house: mineral wool between the rafters, PUR with VCL below.

    BUT, BUT: worth finding out what the bitumen/felt is. Is it at all vapour open? If not then any sort of insulation directly under the boards is questionable IMHO, you'd definitely need good ventilation there.
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